1. Field of the Invention
This invention relates to a range finding device, and, more particularly, it is concerned with the range finding device of a type such that a distance to an object is measured by detecting electrically a relative quantity of discrepancy in space between two images in utilization of the distance measurement principle in a base line double-image coincidence type distance measurement meter. More specifically, the invention relates to a range finding device of a type, in which image scanning signals on the two images are obtained by scanning the abovementioned two images, based on which signals a relative quantity of spatial discrepancy between the two images may be detected.
2. Description of the Prior Art
There have so far been proposed various types of electric double-image coincidence detection type range finding devices as outlined in the preceding, or automatic focus adjusting devices for photographic cameras, etc. using such range finding devices. More specifically, this range finding device is of such type that two images of an object to be formed by a range finding optical system with a relative positional parallax corresponding to the object distance are received by photo-electric light receiving means, and quantities of relative positional parallax in these two images are found from outputs of the light receiving means obtained at this time, thereby calculating a distance to the object for the range finding.
For example, according to Japanese Patent Publication No. 48-5733 (published Feb. 20, 1973, for the invention of "An Automatic Focus Adjusting Device in a Camera" in the name of Canon Kabushiki Kaisha), there is proposed a range finding device in the form of an automatic focus adjusting device, wherein a pair of photo-conductive elements which are so constructed that their resistance values may vary in accordance with positional changes in an image on the light receiving surfaces thereof are juxtaposed, and then images of the same object are formed on these elements by means of a range finding optical system comprising a pair of focussing lenses fixedly disposed at a certain length on the base line so that a distance to the object may be detected by finding a difference in the resistance values between these two elements by utilizing the principle that a relative quantity of discrepancy in the image forming position on each element corresponds to a distance to the object.
In this disclosed device, however, the photo-conductive element per se is of a very peculiar structure, on account of which there inevitably take place various inconveniences in using a pair of these elements such that coincidence of the response characteristics in both elements in an ideal state becomes highly difficult, as the result of which precision in the detection becomes deteriorated due to increase in error signals, and false determination in distance will arise in the range finding due to coincidence of the resistance values in both elements in spite of a discrepancy in the relative positions of the images on both elements.
With a view to solving such problem, there has been proposed a range finding device, or an automatic focus adjusting device, in which image scanning signals on the two images are obtained by scanning them in utilization of a photo-electric light receiving means, and then by finding a quantity of relative positional parallax in these two images from the two images scanning signals, the object distance is calculated, or the focus adjustment of the objective lens system in a camera with respect to the object is automatically attained by utilizing informations concerning a relative positional parallax in these two images.
For example, U.S. Pat. No. 3,898,676 (filed Dec. 20, 1975, granted to Hosoe et al. for an invention entitled "Distance Detecting Device" assigned to Canon Kabushiki Kaisha) teaches an automatic focus adjusting device of a construction, wherein arrays of photo-sensors are used as the photo-electric light receiving means for receiving the two images, and, by driving these photo-sensor arrays simultaneously, a photo-electric output of each photo-sensor in the arrays is obtained in a timed sequence to thereby scan the two images simultaneously in a purely electrical manner, and image scanning signals to be obtained on these two images at this time are converted to wave form signals through low pass filters, after which these wave form signals are introduced into a phase discriminator to detect a phase difference between these image scanning signals, and a servo-motor is actuated by an output from the phase discriminator to cause the objective lens system to shift along its optical axis, in association with which one of the two images is shifted with respect to the other, whereby "in-focus position" of the objective lens system to the object is determined with a point where the phase difference between the image scanning signals for the two images becomes zero, in other words, a point where the relative positional parallax of the two images become zero.
Also, according to Laid-Open Japanese Patent Application No. 51-45556 (Laid-open on Apr. 19, 1976 for the invention of "Method and Apparatus for Distance Detection"), there is proposed a method and an apparatus for detecting distance of an object, which is constructed in such a manner that self-scanning image sensors (a kind of photo-sensor array) are utilized as the photo-electric light receiving means for receiving two images, that the two images are repeatedly scanned by these image sensors, at which time coincidence and non-coincidence of the image scanning signals on the two images to be obtained from the image sensors are detected by means of a coincidence detection circuit, while a timing for commencing scanning of one of the image sensors is varied by a variable delay circuit against a timing for commencing the scanning of the other image sensor, and that the relative positional parallax quantities of the two images, i.e., the object distance is made known directly from a lagged quantity between the timings for starting the scanning operations by the two image sensors which has been found upon detection of the coincidence of the abovementioned image scanning signals by the abovementioned coincidence detection circuit.
In the devices and methods as disclosed in these U.S. Pat. No. 3,898,676 and Laid-Open Japanese Patent Application No. 51-45556 there are used, as the photo-electric light receiving means, photo-sensor arrays or image sensors known as, in particular, Photo-Diode Array, CCD (Charge Coupled Device), BBD (Bucket Brigade Device), and so forth in order to obtain an image scanning signal relative to the two images by scanning the two images of the object to be formed by the range finding optical system. Although these photo-sensor arrays or image sensors have become widely available at a relatively cheap price thanks to recent development in the semiconductor technology, they are still at an expensive level in the overall manufacturing cost, so that the devices incorporating these photo-sensor arrays or image sensors are not yet free from the practical disadvantage of high price. In these devices, if there exists non-uniformity in the characteristic properties of the multitude of the photo-sensors in the photo-sensor arrays or image sensors to be used, such non-uniformity constitutes the direct and primary cause for decreasing precision in the range finding operation. Each and every photo-sensor in the photo-sensor arrays or image sensors should have a uniform and high quality characteristic, which would, however, lead to a further increase in the price of the apparatus.
In contrast, as taught in U.S. Pat. No. 3,553,455 (filed Feb. 26, 1969, granted to Ohtake et al. for the invention entitled "Automatic Focusing Apparatus by Means of Phase Discrimination"), if the two images are scanned by the use of a mechanical vibrating slit, it is only sufficient that a single photo-electric element is made to correspond to each image with the consequence that the inconvenience following the use of the photo-sensor arrays or image sensors as described above can be eliminated. Any way, these devices as heretofore been proposed are of such type that any phase difference in the image scanning signals relative to the two images is detected simply by the phase discriminator upon obtaining of the image scanning signals, so that these devices are considerably devoid of fidelity in range finding precision, hence the focus adjusting operations inevitably become inaccurate, even when they are adopted as the automatic focus adjusting device in photographic cameras, and also, from the standpoint of the construction of the signal processing circuit, there arise various practical inconveniences. For example, in the device proposed in the above-described U.S. Pat. No. 3,898,676, as already mentioned above, the image scanning signals are converted to the waveform signals by causing them to pass through low pass filters, after which the waveform signals are introduced into the phase discriminator, where detection is conducted to find whether any phase difference has occurred between the two signals. In this case, since the phase discriminator deals with the waveform signals, in particular, the overall construction of the device becomes complicated, and, since its reliability is lacking, it is not possible to accurately detect the phase difference when the phase difference is extremely small, on account of which the operation of the phase discriminator as the automatic focusing device is unavoidably inaccurate. Similarly, in the device as proposed in U.S. Pat. No. 3,553,455, too, the image scanning signals to be obtained from the abovementioned photo-electric elements are converted to rectangular waveform signals by amplifying them to a saturation level by means of amplifying circuits, and then, by processing the rectangular waveform signals on the basis of a reference signal associated with vibration of the abovementioned vibrating slit by means of a phase discriminating circuit consisting of a combination of differential circuits, "AND" gates, and "NAND" gates, a signal corresponding to the phase advance or phase delay is obtained. In this device, however, as the differential circuit is particularly used in the construction of the phase discriminating circuit, the precision in the phase discrimination cannot be satisfactorily guaranteed, so far as accurate "rectangular" waveform signals exactly corresponding to the patterns of the two images are not obtained by the abovementioned amplifying circuit. As the result, in the light of the fact that such accurate "rectangular" waveform signals are practically very difficult to obtain, the phase discriminator is lacking its reliability in the precision of its phase discrimination, and, in particular, from the standpoint of the problems such as noise signals, etc. which occur unavoidably at the time of converting the image scanning signals on the two images to the "rectangular" waveform signals. Incidentally, in the device as taught in this U.S. Pat. No. 3,553,455, the signals corresponding to the advance phase or the delay phase to be produced as the outputs from this phase discriminating circuit are used, as they are, as the signals for the forward and reverse rotations of the servo motor to thereby cause one of the two images to shift with respect to the other by this servo motor until the phase discrepancy can be eliminated. At this time, by axially shifting the objective optical system in association with the image shifting, the automatic focus adjustment in the objective lens system can be attained. According to this phase discriminating circuit construction adopted in this prior art patent, the relative positional parallax of the two images is eliminated, and both forward rotation signal and reverse rotation signal are simultaneously imparted to the servo-motor upon elimination of the phase discrepancy between the image scanning signals with the consequence that there takes place such inconvenience that the adjustment in the objective lens system becomes uncertain due to the so-called "hunting" phenomenon which occurs in the vicinity of the "in-focus position" of the objective lens system.
Incidentally, in the device as proposed in the above-discussed Laid-Open Japanese Patent Application No. 51-45556, there is merely adopted a differential amplifier or a combination of the differential amplifier and a comparator as the circuit for detecting coincidence and non-coincidence of the two image scanning signals. Considering, however, that the signals which the circuit deals with are time sequential signals to be produced, as an output, from the image sensors, it is almost impossible to carry out detection of the coincidence and non-coincidence of the image scanning signals with such simple circuit construction. Accordingly, it is not at all possible to expect such distance detection of high precision.
To add more, in the device as proposed in the above-discussed U.S. Pat. No. 3,553,455, since the two images are scanned by utilizing the vibrating slits, it has a definite advantage in that the light receiving means as required can only be a pair of photo-electric elements. However, as has been well recognized in the field of optical technology in general, when the slit is used, an effect due to its diffracted light becomes a large problem, so that, with this proposed device, there exists such inconvenience that deterioration in the range finding precision cannot be avoided owing to possible mixing of noise signals in the output signals in each photo-electric element caused by this diffracted light.